1. Field of the Invention
The present invention relates to the manufacturing of semiconductor devices. More particularly, the present invention relates to a polishing pad of CMP equipment for use in polishing and planarizing the surface of a semiconductor wafer.
2. Description of the Related Art
Semiconductor devices comprise a plurality of circuit patterns stacked one atop the other on a wafer. The circuit patterns are formed by selectively and repeatedly performing numerous unit processes such as photolithography, etching, ion implantation, diffusion, and metal deposition processes. Recently, interlayer circuit patterns are being overlaid with greater precision, and the line widths of the circuit patterns are being made smaller to meet the demand for more highly-integrated semiconductor devices. Moreover, the forming of such circuit patterns involves depositing or growing different layers of materials one atop the other on a wafer. As a result, the surface of the wafer becomes uneven. If not attended to, the uneven surface would cause alignment errors in a photolithography process, for example, whereby process failures would occur. In view of this, the wafer needs to be planarized between successive ones of the unit processes.
One known process of planarizing a wafer is chemical mechanical polishing (CMP). The CMP process employs a polishing pad to polish and planarize the surface of the wafer during the fabrication of semiconductor devices.
As shown in FIG. 1, a polishing pad 10 used in a typical polishing process is adhered to the top of a plate 12, which is rotated at a high speed. A slurry S is supplied towards a central region of the polishing pad 10. The slurry should be uniformly distributed across the upper surface of the polishing pad 10 by centrifugal force. At the same time, a wafer W adhering to a polishing head 14 is pressed by the head 14 against the surface of the polishing pad 10, is rotated at a high speed, and is moved across the polishing pad 10 between the central region of the polishing pad 10 and the outer peripheral region thereof. The wafer W is maintained parallel to the surface of the polishing pad 10, i.e., is maintained in a horizontal orientation, by a gimbal 16 of the polishing head 14.
As described above, the main purpose of polishing the wafer W is to planarize the surface of the wafer W. Therefore, the surface of the polishing pad 10 must be continuously maintained flat and even, and the slurry S must be uniformly distributed across the surface of the polishing pad 10.
The surface state of the polishing pad 10 is maintained by a conditioning head 18, which is located at one side of and above the polishing pad 10. The conditioning head 18 is driven to cut the surface of the polishing pad 10 during the polishing process or periodically. Furthermore, as shown in FIG. 2, the slurry S is distributed uniformly across the surface of the polishing pad 10 by a groove G, as the slurry S flows form the center of rotation of the polishing pad 10 to the outer periphery thereof under the centrifugal force imparted to the slurry S due to the high rotational speed of the plate 12. Furthermore, the polishing pad 10 comprises polymeric material having micro-cavities B. The micro-cavities B are exposed over the entire surface of the polishing pad 10, as shown in FIGS. 3 and 4. Accordingly, the slurry S flows in and out of the micro-cavities B as well as the grooves G so as to be distributed uniformly between the polishing pad 10 and the wafer W.
Now, the structure of the polishing pad 10 will be explained in more detail with reference to FIG. 3. The polishing pad 10 is a structure having multiple layers whose physical properties differ from each other. The multiple layers include a polishing layer 10a that faces the wafer W during the polishing process, a support layer 10b adhered to the top of the plate 12, and an epoxy layer 10c interposed between the support layer 10b and the polishing layer 10a to bond the two layers 10a and 10b. The polishing layer 10a is typically formed of a polyurethane material having micro-cavities B. The surface of the polishing layer 10a is maintained in tight contact with the surface of the wafer W because the micro-cavities B exposed at the top surface help impart an elastic and flexible state to the top portion of the polishing layer 10. The support layer 10b has a porous structure, which is more elastic and flexible than the polishing layer 10a, and is compressed by the gimbal 16 of the polishing head 18 and the wafer W held thereby. The support layer 10b thus urges the polishing layer 10a back into its initial state.
During the polishing process, some of the slurry S that has flowed across the polishing pad 10 is flung off the pad by centrifugal force. However the rest of such slurry flows from the surface of the polishing layer 10a down along the sidewall thereof. The slurry S reaches the support layer 10b, which is relatively thick, and penetrates into the support layer 10b through its pores. The penetration of the slurry S into the support layer 10b damages the adhesiveness between the support layer 10b and the plate 12. As a result, the support layer 10b and the plate 12 separate at their interface, and the elasticity of the support layer 10b decreases at regions of the interface filled by the slurry S.
To avoid this potential problem, the adhesive strength between the support layer 10b and the plate 12 could be increased. However, such a measure would make it difficult to separate the polishing pad 10 from the plate 12 when replacing the worn polishing pad 10. Furthermore, such a measure would give rise to many other problems such as the long time it would require to clean the surface of the plate 12 after the worn polishing pad was removed therefrom.
Regardless, the support layer 10b should be very tightly adhered to the surface of the plate 12. However, even if specific efforts were taken to tightly adhere the support layer 10b to the plate 12, local air spaces would still be present therebetween because the support layer 10b is of a porous flexible material. The air spaces adversely affect the elasticity of the support layer 10b, which can result in failures in the process of polishing the wafer W. This phenomenon may last even after the surface of the polishing pad 10 is conditioned by the conditioning head 18.
Furthermore, the slurry has been found to penetrate the support layer 10b even as far as a detecting unit 20 for detecting the degree to which the wafer W has been polished. The detecting unit 20, as shown in FIGS. 2 and 4 comprises a detecting window 20a. A predetermined local region of the polishing pad 10 is cut out from the polishing layer 10a down to the plate 12, the projection window 20a is inserted into the cut-out region, and the projection window 20a is adhered to the layers of the pad. The detecting unit 20 also comprises a photo detector 20b, 20c disposed on the plate 12. The photo detector 20b, 20c intermittently emits probe light onto the surface of the wafer W through the projection window 20a, and collects the light reflected from the surface of the wafer W.
The projection window 20a must be tightly adhered to the polishing pad 10. However, if the projection window 20a is not strongly adhered to the polishing pad 10, and the area of contact area between them is deformed by the downward force exerted on the polishing pad 10 by the polishing head 14 and the wafer W, the projection window 20 may separate from the polishing pad 10. As a result, some of the slurry S flows into the support layer 10b through the area of separation between the projection window 20 and the polishing pad 10.
As a possible countermeasure, the adhesive strength between the polishing pad 10 and the projection window 20a could be enhanced. However, in this case, the physical property of the area of contact between the polishing pad 10 and the projection window 20a would differ significantly from those portions of the pad 10 around the cut-out area. The portion of the surface of the wafer W passing over the region of the polishing pad corresponding to the cut-out would be polished to a degree different from other portions of the wafer surface.
Still further, the surface of the support layer 10b of the polishing pad 10 loses its elasticity and forms more and more dimples over time because it is continuously compressed by the polishing head 14. As a result, the polishing layer 10a of the polishing pad 10 needs to be cut frequently by the conditioning head 18 to eradicate the dimples, thereby reducing the useful life of the polishing pad 10.